Telemetry, Logging, and Flow Export

Synopsis #

This chapter provides operational observability patterns for OpenBSD firewalls and routers: packet-filter logging via pflog(4) and pflogd(8) , flow export with pflow(4) to external collectors (NetFlow v5/v9 and, where supported by your release, IPFIX), system and service telemetry with snmpd(8) , and central log shipping with syslogd(8) configured through syslog.conf(5) . Verification relies on pfctl(8) , tcpdump(8) , and systat(1) . Use these patterns to produce actionable signals for incident response, capacity planning, and security monitoring.

Design Considerations #

Clocks and identity. Accurate time is mandatory for correlation. Run ntpd(8) and ensure unique hostnames and stable IP sources for telemetry.
Scope and privacy. Only log and export what you can store and protect. Avoid full packet payloads unless explicitly justified. Prefer sampled or flow-level telemetry for aggregate visibility.
Separation of concerns. Treat the firewall/router as a producer. Perform heavy analytics, long-term storage, and dashboards on external systems.
Network paths. Place collectors on dedicated management networks or via isolated VPNs. Protect UDP-based feeds (for example, NetFlow) from loss and spoofing.
Version caveats. pflow(4) supports NetFlow v5/v9. IPFIX availability and options are release-dependent. Confirm capabilities on OpenBSD 7.8 before enabling specific templates.
Rotation and retention. Size /var and set appropriate rotation in newsyslog.conf(5) . Ship logs off-box; do not rely only on local disks.

Configuration #

1) Packet filter logging (pflog) #

Enable targeted logging in pf.conf(5) . The log keyword on a rule emits metadata to pflog0, which pflogd(8) stores as a rotating pcap at /var/log/pflog.

## /etc/pf.conf — excerpt with rule logging

set skip on lo

wan = "em0"
lan = "em1"
lan_net = "10.10.10.0/24"

block all

# Log blocked unsolicited WAN traffic
block log on $wan

# Log SSH admin access attempts to the firewall
pass  log on $wan proto tcp to ($wan) port 22 keep state

# Allow and log HTTPS from LAN to anywhere (for demo; tighten for production)
pass  log on $lan proto tcp from $lan_net to any port 443 keep state

Apply the policy and ensure pflogd is enabled:

# pfctl -f /etc/pf.conf
  # Atomic reload of PF rules
# rcctl enable pflogd
  # Start pflog capture on boot
# rcctl start pflogd
  # Launch now; writes /var/log/pflog
# tcpdump -n -e -ttt -i pflog0
  # Live view of PF log stream (Ctrl-C to stop)

Use pfctl -vvsr to see which rules carry log and to inspect per-rule counters.

2) Flow export with pflow(4) (NetFlow v5/v9, optionally IPFIX) #

pflow(4) exports summaries of PF state traffic to a remote collector over UDP. Configure a pflow0 interface with a stable source address, collector destination (IP:port), and protocol version.

Assumptions:

Export source: 192.0.2.10 (loopback or a management address)
Collector: 192.0.2.50:2055 (NetFlow)
Protocol: v9

# ifconfig pflow0 create
  # Create the exporter interface
# ifconfig pflow0 flowsrc 192.0.2.10 flowdst 192.0.2.50 2055 flowproto 9
  # Set source, destination, and NetFlow version
# ifconfig pflow0 up
  # Activate exporter
# ifconfig pflow0
  # Verify parameters

Permit the export path in PF (replace em0/addresses as needed):

## /etc/pf.conf — allow pflow export to collector

set skip on lo
wan = "em0"

pass out on $wan proto udp from 192.0.2.10 to 192.0.2.50 port 2055 keep state

Reload and confirm with a packet capture toward the collector:

# pfctl -f /etc/pf.conf
# tcpdump -ni $wan host 192.0.2.50 and udp port 2055
  # Observe flow export packets; verify at the collector side as well

If your collector expects IPFIX, check pflow(4) on OpenBSD 7.8 for IPFIX support and template options, then switch flowproto accordingly.

3) System telemetry with snmpd(8) #

snmpd(8) exposes standard MIBs (interfaces, system, sensors). Bind it to a management interface and restrict access. The exact model (community vs. SNMPv3) depends on your security policy; SNMPv3 is recommended where supported by your tooling.

Minimal example: listen on LAN and allow read-only queries from 10.10.10.0/24.

## /etc/snmpd.conf — minimal read-only service

listen on 10.10.10.1

system contact "netops@example.com"
system location "DC1-R1"

read-only community "monitoring" source 10.10.10.0/24

# rcctl enable snmpd
# rcctl start snmpd
# pkill -0 snmpd && echo "snmpd running"
  # Basic liveness check

Consult snmpd.conf(5) for SNMPv3 user configuration and trap receivers. Ensure PF permits UDP 161 from your NMS and any trap egress (UDP 162).

4) Central log shipping with syslogd(8) #

Forward local logs to a remote collector using syslog.conf(5) . UDP forwarding uses a single @ before the host.

## /etc/syslog.conf — forward everything to a collector and keep local files

*.*     @192.0.2.60
# Keep existing local file rules below (trim to your needs)
auth.info       /var/log/authlog
daemon.info     /var/log/daemon
kern.info       /var/log/kernlog
pf.info         /var/log/pflogdump

# rcctl reload syslogd
  # Re-read configuration; syslogd sends to 192.0.2.60:514/udp
# logger -t test "syslog forwarding test $(date)"
  # Send a test message; verify it arrives at the collector

If your collector requires TCP or TLS, place a log relay in front of it or consult the collector’s ingestion options. Keep /etc/newsyslog.conf tuned for local retention.

Verification #

PF rule counters and logs with pfctl(8) and live pflog:

$ pfctl -vvsr | egrep 'log\b'
  # Confirm which rules log and observe counters increasing
$ tcpdump -n -e -ttt -i pflog0
  # Live packet logs for validation during tests
$ tcpdump -nr /var/log/pflog | head
  # Inspect the rotated capture file

pflow export on the wire:

# tcpdump -ni em0 host 192.0.2.50 and udp port 2055
  # Confirm NetFlow/flow export packets leaving the router

SNMP reachability (from NMS or a test host):

$ snmpwalk -v2c -c monitoring 10.10.10.1 sysUpTime.0
  # Basic liveness; substitute v3 commands if applicable

syslog forwarding path:

$ logger -t probe "forward-test $(date)"
  # Emit a message and confirm it arrives at 192.0.2.60
$ tail -n 5 /var/log/messages
  # Local copy remains available per syslog.conf

Dashboards: use systat(1) pf and ifstat modes during rollout to correlate queueing, rule hits, and interface rates with exported telemetry.

Troubleshooting #

No pflog records. Ensure rules contain log, PF is loaded, and pflogd(8) is running. Use tcpdump -i pflog0 to bypass files and validate the stream.
Collector does not see pflow. Verify flowsrc/flowdst/flowproto on pflow0, that PF permits UDP to the collector, and that intermediate ACLs allow it. Confirm time sync; some collectors drop excessively skewed timestamps.
High pflow loss. Flow export is best-effort over UDP. Reduce sampling window (where supported), dedicate a management path, or deploy a closer collector. Ensure CPU is not saturated during bursts.
SNMP timeouts. Confirm listen on address and source restriction match the NMS. Verify PF allows UDP 161 from the NMS and that community or v3 credentials are correct.
Syslog not forwarding. Check syntax in /etc/syslog.conf, reload syslogd, and confirm reachability to the collector’s UDP 514. If the collector requires TCP/TLS, use its supported ingestion method.
Disk pressure in /var. Increase rotation frequency in newsyslog.conf(5) and export logs/flows off-box. Consider excluding noisy facilities or raising thresholds.